First generation photovoltaics, single layer p-n junction diodes (silicon wafer-based solar cells) are the dominant technology in the commercial production of solar cells, accounting for more than 86% of the solar cell market. Between 2000 and 2004, the increase in worldwide solar energy capacity was an annualized 60%. Although 2005 was expected to see large growth again, shortages of refined silicon started hampering production worldwide in late 2004.
Multi-junction thin film technology for the second generation photovoltaics was aimed to fit the solar spectrum better and reduce the cost (a-silicon, poly-crystalline silicon, microcrystalline silicon, CdTe, copper indium selenide/sulfide CIGS). However neither of these goals has been achieved.
All solar cells of these two generations are based on the principle that the quantum efficiency (QE) equals one for photons with the energy above the bandgap (hv>Eg). Very recently, it was demonstrated that a photon with the energy hv>3Eg can create more than one electron-hole pair (exciton) via impact ionization—carrier multiplication (CM). Up to 7 pairs were demonstrated being generated in the time frame of ˜10-12 fs in ultra-small nanocrystalline (˜10 nm) lead salt PbS and PbSe materials. With CM quantum efficiency increases roughly linearly up to 700% with pump photon energy above the activation threshold [3] with theoretical limits for energy efficiency of >60% without CM. However, practical realization of these principles requires very fast separation of electrons and holes to prevent their recombination.